The present invention relates to the field of high-voltage capacitors and in particular, but not exclusively, to fixed and variable capacitors for use in high-power radio-frequency applications.
Common applications of high-voltage capacitors include broadcasting (eg in an oscillation circuit of a high power transmitter) or plasma controlling processes in the semiconductor, solar and flat panel manufacturing equipment (in so-called impedance matching networks). The term “high voltage” is used in the context of this description to refer to capacitor applications in which the voltage applied across the capacitor electrodes exceeds 1000 volts. Indeed, some high-voltage capacitors may be required to have breakdown voltages in excess of 10 kV. Typical operating parameter ranges for such capacitors might be, for example, an applied voltage between 10 kV and 20 kV, device current between 30 A and 150 A, operating frequencies of between 5 MHz and 30 MHz and capacitance values of between 5 pF and 5000 pF.
Such high voltage capacitors are commonly manufactured using a vacuum as the dielectric, in order to achieve the high breakdown voltage which is required. Vacuum capacitors are known for their high resistance to breakdown and for their thermal stability in operation. The vacuum chamber is pumped down to a very low pressure (typically lower than 10−6 mbar) and kept low over the entire lifetime of the device, which may be many years, by the vacuum-tight enclosure. The vacuum ensures good electrical insulation between the electrodes and very low dielectric losses in the device.
Manufacturers of high-voltage capacitors have concentrated on achieving the best possible and longest-lasting vacuum, and have had significant success in manufacturing vacuum capacitors which offer very high operating voltages (even at high operating currents and high frequencies), in a relatively small device package. However, the manufacture of such capacitors is complex, requiring high vacuum pumps, high quality materials and a high-quality welding or brazing process to join the end-caps to the (usually ceramic) capacitor walls in a vacuum-tight way. The specific joining method also limits the choice of the materials used for manufacturing the end-caps and/or electrode material, since some materials are difficult to braze or weld.
An example of a vacuum capacitor can be found in British patent GB748560, which also discloses the use of SF6 as a gas dielectric, at pressures of 30 to 40 pounds per square inch (2.07 to 2.76 bar). In order to use such a capacitor for high voltages (eg 10 kV or more) with a gas dielectric at 2.76 bar, the electrode spacing would need to be large, and the overall size and weight of the device would be large as a result. However, SF6 has been identified as a greenhouse gas, and the manufacture and disposal of such capacitors therefore entails significant extra complexity and cost in order to avoid emission of SF6 into the atmosphere.
It is desirable to address at least some of these and other problems with prior art devices. It is desirable to provide a high-voltage capacitor which has comparable voltage and current handling characteristics to a prior art vacuum capacitor of similar size and capacitance, but which:                is simpler to manufacture and assembly,        requires less maintenance and monitoring,        has an increased serviceable lifetime,        is less susceptible to failure if the pressure inside the device changes, and/or        permits the use of a more environmentally-friendly gas as the high-voltage dielectric.        
In particular, the invention foresees a high-voltage capacitor comprising a gas-tight enclosure, the gas-tight enclosure containing a gas dielectric and at least two capacitor electrodes, wherein the pressure of the gas dielectric in the gas-tight enclosure is at least 6 bar. It is desirable to provide a method of manufacturing such a capacitor, the method comprising an assembly step of assembling the gas-tight enclosure, and a pressurizing step of filling the gas-tight enclosure with the gas dielectric to the said pressure of at least 6 bar.
According to another variant of the invention, the gas-tight enclosure comprises an insulating body section and two end caps. The insulating body section may be made at least partially of a polymeric material. The polymeric material is preferably PEEK and/or a polymeric material which is reinforced with reinforcement fibres. Such materials are strong enough in tension to withstand the elevated interior pressure, are easily machined with adequate precision, and/or do not interfere electrically with the operation of the capacitor.
According to a further variant of the invention, the pressure of the gas dielectric is at least 10 bar, or at least 15 bar. The elevated pressure inside the device allows the capacitor to be used at high voltages, as will be discussed in relation to FIG. 2.
According to another variant of the invention, the gas-tight enclosure comprises a substantially cylindrical chamber having an axial length of between 25 mm and 200 mm and/or a diameter of between 35 mm and 150 mm.
According to another variant of the invention, the breakdown voltage of the capacitor is at least 10 kV and/or the capacitance of the capacitor is between 5 pF and 5000 pF.
According, to another variant of the invention, the electrodes are made of aluminium or an alloy comprising a majority of aluminium.
According to another variant of the invention, the electrodes are formed using a non-conducting or poorly-conducting material coated with a highly-conducting material.
According to another variant of the invention, the electrodes are formed as concentric cylinders or as interleaved spirals.
According to another variant of the invention, at least one of the end caps is formed contiguously with at least one of the electrodes, from a single block of metal.
According to another variant of the invention, at least one of the end caps is secured to the insulating body section by a threaded joint.
According, to another variant of the invention, at least one of the end caps is secured to the body section by a positive-lit joint, a bayonet-fitting, by an adhesive, by welding or by brazing or soldering.
According to another variant of the invention, the gas is one of air, N2 or other inert gas or mixture of inert gases. N2 is preferred, since it is safe, easily available and offers good, consistent breakdown characteristics.
According to another variant of the invention, a quantity, referred to as F, is greater than 0.02, where F=VB×C/V, where V is the internal volume of the gas-tight enclosure in mm3, where C is the capacitance of the capacitor in pF and where VB is the breakdown voltage of the capacitor in kV.
The figures are provided for illustrative purposes only, and should not be construed as limiting the scope of the claimed patent protection. For example, the example capacitor shown in the FIGS. 1a to 1c is a fixed capacitor, but it will be evident to the skilled person that the invention can also be implemented in a variable capacitor.
Where the same references have been used in different drawings, they are intended to refer to similar or corresponding features. However, the use of different references does not necessarily indicate a difference between the features to which they refer.